1. Field of the Invention
This invention relates generally to apparatus and methods for converting direct current to alternating current and particularly to an inverter which incorporates pulse width modulation to reduce the magnitude of undesirable harmonics.
2. Description of the Prior Art
Many direct current to alternating current inverter systems use pulse width modulation to control both single phase and polyphase outputs. There is increasing use of solid state inverters for control elements, particularly in applications to variable frequency A.C. motor drive systems. One of the desirable characteristics of an inverter drive system for such an application is the ability to maintain constant motor torque over a relatively wide motor speed range, which requires that the A.C. motor energizing potential display a substantially constant ratio of voltage to frequency (or volts/Hertz) as the frequency varies. Most solid state controls maintain a fixed ratio of voltage to frequency by either controlling the D.C. input to a fixed pulse width inverter or by controlling the inverter output pulse width as a function of frequency and using a substantially constant voltage source of D.C. energy.
Because the first method mentioned usually involves two stages of power control, recent variable speed motor drive systems use the pulse width modulation (PWM) method to achieve the necessary control characteristics. One of the disadvantages associated with operating a variable frequency multiphase inverter using PWM techniques is an increase in the magnitudes of undesirable harmonics in the output waveforms. The presence of undesirable harmonics in the output waveform is particularly important in the lower range of the operating frequency spectrum wherein such harmonics usually increase in amplitude compared with the amplitude of the fundamental frequency component created under such conditions.
The prior art contains many examples of pulse width modulated power sources capable of producing various waveforms. The "six-step" waveform is the basic waveform of many prior art inverters. The conventional six-step waveform results from switching each of the three legs of a three-phase inverter twice in each wavelength of the output fundamental frequency. A sequence of switching actions occurs with a phase difference of 60 degrees separating the switching action in each leg from the switching in the other two legs. The potential differences between pairs of legs of the inverter produce the three-phase output waveforms. While it is generally accepted that the six-step wave is satisfactory for energizing an A.C. motor, modulation of the six-step waveform for variable frequency A.C. motor drive systems produces outputs which are relatively high in harmonic content. It is possible to use PWM to control the amplitude of the fundamental frequency component of a six-step waveform; however, as the amplitude of the fundamental frequency component decreases, the amplitudes of the fifth and seventh harmonics, which are the most undesirable, increase to such portions of the total waveform that motor losses severely penalize the effectiveness of the system.
In the prior art, the effects of increasing amplitudes of the fifth and seventh harmonics are minimized by using a multiple inverter system and combining the outputs of the inverters such that the undesirable harmonics are substantially eliminated. Multiple inverter systems usually introduce considerable complexity and cost into the inverter equipment.
A second method for minimizing undesirable harmonics in the output of an inverter system while modulating the fundamental frequency component is to introduce additional switching actions into the inverter to produce notches in the output waveforms. The placement and control of auxiliary switching points has been the subject of considerable investigation which has produced numerous switching systems. In general, the goal of each system is to minimize the number of inverter switching actions consistent with the capability to control the amplitude of the fundamental frequency component and harmonic content over a wide operating frequency range. However, motor losses due to fifth and seventh harmonics has heretofore remained a serious difficulty in variable frequency A.C. motor drive systems.
The art exhibits a need for a low-cost, efficient inverter system which produces a variable frequency output having a constant volts/Hertz in the fundamental frequency and having acceptable fifth and seventh harmonic frequency components.